1. Field of Invention
The present invention relates to well boring in general and in particular to a method and apparatus for sensing a pipe joint within a well structure.
2. Description of Related Art
In hydrocarbon production, a well may be formed by an outer casing located within a wellbore and may optionally be surrounded by cement. The well may then include a tool or production string therein for working or producing from the well. Due to the potentially high pressures within the well from hydrocarbons extracted from the hydrocarbon producing formation, numerous types of shut-off valves, spools and other fittings to isolate and control access to the well, such as, by way of non-limiting example a Christmas tree, as it is commonly known or a snubbing rig.
The well structure may include shut-off valves for closing off or otherwise completely or partially sealing the top of the well as desired by a user. In particular, one common design for such valves are pipe rams which utilize a pair of opposed rams which are movable along a plane perpendicular to the well bore. The rams may be moved along the plate by pistons or the like and are operable to be moved out of the central passage of the well or to be pressed together to seal the well. Rams may be of a blind or shear type to completely seal the well or of a pipe ram type in which the two rams each include a half-circle hole sized to pass a pipe therethrough when the two rams are pressed together. Such pipe rams are commonly utilized in snubbing rigs to seal around the drill or production string and isolate the well below the pipe ram from the environment while permitting the drill or production string to remain within the well or to be extracted or inserted into the well.
One difficulty that exists with common hydrocarbon wells is the difficulty of determining the location of the joints on the tool or production string. Such strings are commonly formed of a plurality of endwise connected pipes which are connected to each other by threaded connectors. Conventionally such threaded connectors are located at each end and provide enlarged portions of the pipe which are strengthened so as to provide a larger stronger section of the pipe to be grasped by tools and the like. Such tool joints present a larger cross-section than the remainder of the pipe. Disadvantageously, such enlarged diameters of tool joints may interfere with the proper operation of pipe rams should the pipe ram be attempted to be closed at the location of such a tool joint or when extracting or inserting the pipe when at least one of the rams is set to hold back the pressure. Such an event is commonly referred to as stripping which may create a risk of the tool joint being pulled or pushed into the closed piper ram thereby damaging the pipe and/or pipe ram.